Adhesive compositions may be applied to a substrate via extruding (e.g., the adhesive is melt extruded onto the substrate, or coextruded with the substrate), blowing (e.g., the adhesive is blown into microfibers), solvent coating (e.g., the adhesive is solublized and/or dispersed in a solvent, applied to a substrate, and the solvent is evaporated off), spraying (e.g., an adhesive is contained in a container and a secondary means of pressure is used to propel the adhesive from the container), or aerosol spraying (e.g., the adhesive and a propellant are enclosed in a common container and the propellant is the sole source of energy to propel the adhesive from the container).
Aerosol spraying of an adhesive presents specific challenges and, therefore, not all adhesive compositions may be used for aerosol spraying. For example, the aerosol adhesive should not only provide adequate adhesion once dispensed, but should flow out of the vessel in a usable spray, should not gum up the delivery mechanism (such as the valve or actuator), and should not cause soak-in of the substrate to which it was applied.
Aerosol adhesives may include a polymer, which along with any glass transition temperature modifiers is the adhesive; a solvent, which acts to dilute or suspend the adhesive; a propellant, which acts to propel the adhesive composition from the containment vessel; and any additional additives. The aerosol adhesive may be classified based on its spray pattern, as: (i) a mist (or particle) spray, (ii) a lace (or web) spray, or (iii) a combination of a mist spray and a lace spray.
Mist sprays deliver small particles of adhesive in a round or fan shape pattern with minimal to no cobwebbing (or strings) observed in the spray pattern and a variable amount of overspray. Mist sprays may be further categorized as a fine mist spray or a coarse mist spray. One skilled in the art would be able to distinguish coarse mist spray from fine mist spray based on the particle size distribution. Mist sprays deposit a uniform coat of adhesive that is not visible through thin materials such as paper. Traditionally, mist sprays produce a drier application of adhesive than lace sprays because less of the aerosol adhesive soaks into the substrate. Mist spray adhesives are typically based on cross-linked polymers, which are not solubilized in the containment vessel (i.e., the polymer is not soluble in the solvent). These cross-linked polymers, therefore, may settle out of solution and the containment vessel often needs to be shaken before use. Shaking of the containment vessel may not be practical when the containment vessel is a large canister weighing several hundred pounds. Recently, mist sprays have been formulated with hydrogenated styrenic block copolymers that are soluble in the solvent. See for example, European Pat. No. 0 616 018 (Nguyen). However, the mist sprays made with hydrogenated styrenic block copolymers typically give a low strength temporary or removable type of adhesive due to the low amount of adhesive delivered out of the containment vessel. Typically, mist sprays have a solids content of adhesive of about 5 to 25% by weight versus total weight.
Lace sprays deliver a string (or cobwebbed) pattern of adhesive in a controlled fan shaped pattern with little or no overspray. Lace sprays may be further categorized as a light lace spray or a heavy lace spray. One skilled in the art would be able to distinguish a light lace spray or a heavy lace spray based on the thickness of the resulting adhesive. Lace sprays give a textured surface of adhesive, generally give a thicker bondline than mist sprays, and are considered a higher strength adhesive than the mist spray adhesives. Lace sprays traditionally are based on soluble polymers such as, e.g., styrene block copolymers (e.g., styrene-butadiene-styrene (SBS), styrene-isoprene-styrene (SIS), styrene-ethylene-butylene-styrene (SEBS), styrene-ethylene-butylene (SEB), styrene-ethylene-propylene-styrene (SEPS), styrene-ethylene-propylene (SEP)), styrene-butadiene rubber, ethylene vinyl acetate, neoprene, and nitrile. Typically, lace sprays have a soluble polymer concentration of less than 25% by weight versus total weight.